Why do people keep talking about
overcentralization?

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It means you need a specific thing to counter another thing or you will lose.

For example, back in Gen 4, it was pretty much a requirement to have a Mamoswine or Weavile to counter Garchomp, and even then it was kinda iffy. Garchomp overcentralized the metagame and thus, got banned.

I was once modded for illegal activity because I made a topic asking for advice on nicknaming my Pokemon.3DS FC: 5043-2277-6391 - THIEF

It means that, due to the over-presence of certain threats in the metagame, people are forced to include certain checks and counters on their teams. If a Pokemon causes this, it is generally banned to prevent overcentralization of the metagame.